Providing a material that can accurately detect the presence of hydrogen or hydrocarbons is desirable. Hydrogen sensitive metals undergo a change (e.g., a change in resistance) that can be detected when interacting with hydrogen. For example, as hydrogen dissociates on a surface of a metal and migrates into the interior of the metal, the electrical resistance of the metal is changed. Usually, this increases the electrical resistance of the metal. Similarly if the hydrogen dissociates on the surface of a metal that is part of an electrical circuit, then the electrical properties of the entire circuit are affected.
Several metals and metal alloys have applications as hydrogen sensitive metals. Palladium and alloys of palladium containing silver (for example, PdAg) are known hydrogen sensitive metals. It is known to use a palladium resistor as a hydrogen sensor. The resistor is formed by depositing palladium on a substrate. As hydrogen is absorbed by the palladium, the resistance of the metal changes. The change in resistivity can then be detected (e.g., by an electrical circuit connected to the palladium resistor). Additionally, it is known to use Pd or PdAg as a gate in a MOSFET or Schottky diode device. The detection of hydrogen by the gate triggers changes in the electronic properties of the device.
In both of these applications, the Pd and PdAg are sensitive to the presence of hydrogen. The use of these materials, however, does have limitations. For instance, in the palladium resistor as the palladium dissociates and absorbs hydrogen, the palladium undergoes a phase transformation. This causes hysteresis. Furthermore, this phase transformation may damage the layer of palladium. Similarly, the PdAg film experiences a phase transformation. This occurs at higher concentrations of hydrogen. The presence of the silver does, however, reduce damage to the film because the film is more resilient.
It is desirable for a hydrogen sensitive metal in the presence of hydrogen to experience a large change in resistivity without undergoing a phase transformation. Furthermore, it is desirable for the change to be repeatable (i.e., the sensing metal can be used for multiple exposures to hydrogen).
This has been considered by Hughes et al., in "Wide Range H.sub.2 Sensor Using Catalytic Alloys," presented at the 183rd Meeting of the Electrochemical Society, May 1993. Hughes et al. used an alloy of palladium and nickel, particularly Pd13% Ni, as a hydrogen sensitive resistor. This material experienced nearly a 10% change in resistance when exposed to an environment consisting of 100% hydrogen. Additionally, the Pd13% Ni alloy did not undergo a phase transformation and is repeatable. The use of Pd13% Ni alloy as a hydrogen resistor, however, is limited because the alloy experiences a small change in resistance at low amounts of hydrogen (e.g., when exposed to an environment consisting of 10% hydrogen, the change in resistance is small, approximately 1%).
Small changes in resistance may also be attributed to fluctuations in temperature. As a result, it is undesirable to use a material which experiences only a small change in resistance (e.g., less than 1%, as for example, exhibited by the Pd13% Ni alloy) when exposed to low amounts of hydrogen because the change of resistance is similar to those changes caused by temperature fluctuations. It would be difficult to determine whether the change in resistance is a result of the presence of hydrogen or a fluctuation in temperature. Unless strict temperature control of the resistor is possible, these materials are not acceptable for detecting small amounts of hydrogen.
An alloy of palladium and chromium, particularly Pd13% Cr, has been tested as a hydrogen sensitive resistor. The Pd13% Cr alloy also did not undergo a phase transformation when exposed to a hydrogen environment. Additionally, the detection of hydrogen is repeatable. The Pd13% Cr alloy, however, experiences only a 1% change in resistance when exposed to an environment of 100% hydrogen. The use of this material in this form would therefore be unacceptable at both low and high concentrations of hydrogen for reasons discussed above.
Further, the number of alloys available for hydrogen detection is limited. Each alloy has its own sensitivity to hydrogen, hydrocarbons, and poisons. It is desirable to have a wide range of alloys available to enable detection of hydrogen bearing gases in a wide variety of environments and temperatures.
Metal alloys containing palladium and titanium are known. For example, U.S. Pat. No. 4,082,900 to Shimogori et al. discloses a Ti--Pd alloy containing 0.1 to 0.2% of palladium. The addition of palladium reduces crevice corrosion and embrittlement by absorbing hydrogen.
U.S. Pat. No. 4,139,373 to Norton discloses a Ti alloy containing another metal such as palladium. The alloy consists of 60 to 94 weight % Ti and 6 to 40 weight % of at least one additional metal which includes palladium. The addition of palladium to the alloy reduces the corrosion rate and improves the electrical conductivity.
U.S. Pat. No. 4,536,196 to Harris discloses a Pd alloy coated with a layer of titanium. A membrane of the alloy with the titanium coating is used for diffusing hydrogen from a mixture of gases.
U.S. Pat. No. 4,666,666 to Taki et al. discloses a Ti alloy having small amounts of Pd (i.e., between 0.005% to 2.0% by weight). The alloy has improved corrosion resistance and improved resistance to hydrogen absorption.
U.S. Pat. No. 4,719,081 to Mizuhara discloses a Pd alloy containing Ti for use in joining ceramic metals. The alloy includes 65 to 98 weight % palladium, 1 to 20% nickel, 0.5 to 20% chromium, 0.5 to 10 weight % Ti or Zr and 0 to 10% molybdenum.
U.S. Pat. No. 4,728,580 to Grasselli et al. discloses an amorphous metal alloy that may contain Pd and Ti. The alloy is used for reversible hydrogen storage.
None of the above mentioned U.S. patents, however, disclose the use of a PdTi metal alloy as a hydrogen sensor.